The present invention relates to a method of adjusting a frequency response in a filter device of three-conductor type which may be used as a band-pass filter for example.
It is known to provide a filter device of three-conductor type which is utilized as a band-pass filter for a microwave range. An example of such a conventional filter device is illustrated in FIGS. 1 and 2. As will be seen in FIGS. 1 and 2, it comprises a lower dielectric substrate. 1 and an upper dielectric substrate 2 which are stacked with respect to each other. Each of the dielectric substrates 1 and 2 may be of dielectric ceramic material having a high dielectric constant and a lower dielectric loss such as BaO--TiO.sub.2, BaO--TiO.sub.2 -rare earth or the like. The lower dielectric substrate 1 is provided with an external ground conducting layer 3 on the peripheral portion and bottom surface thereof. Similarly, the upper dielectric substrate 2 is provided with an external ground conducting layer 4 on the peripheral portion and upper surface thereof. On the upper surface of the lower dielectric substrate 1 are disposed a plurality of stripline resonator conducting layers 5, 6 and 7 which form a filter element. Each resonator conducting layer has one end or an open circuit end (5a, 6a and 7a ) spaced from the ground conducting layer 3 and the other end or a short circuit end (5b, 6b and 7b) connected to the ground conducting layer 3. The open circuit ends 5a, 6a and 7a of the respective resonator conducting layers 5, 6 and 7 are alternately disposed so as to form an interdigitated configuration. The upper dielectric substrate 2 is fixed on the lower dielectric substrate 1, and the ground conducting layers 3 and 4 of the respective dielectric substrates are connected to each other.
As well known in the art, the filter device of this type has a frequency response which depends on the the configuration and dielectric constant of the substrates, and the dimension of the resonator conductors. Upon the manufacturing of the filter device the dielectric constant of the substrates and the size of the resonator conducting layers are strictly determined. However, it can not be avoided that there may occur deviations in the dielectric constant of the substrates and in the dimension of the resonator conducting layers. It is, therefore, necessary to adjust the frequency response of the filter device after being completed.
The adjustment of the frequency response can not be performed by adjusting the length of the resonator conducting layers because they are embeded in the dielectric substrates. One solution to this problem has been proposed in U.S. Pat. No. 4,157,517. According to the adjusting method disclosed in this U.S. Patent, the frequency of the filter is previously set at a lower level than a desired one, and the external conductor or ground conducting layer 4 provided on the upper surface of the upper substrate 2 is partially removed at regions 8 adjacent the open circuit ends of the resonator conducting layers 5, 6 and 7 to reduce the capacitance between the external conducting layer 4 and the respective resonator conducting layers and to increase the response frequency of the filter thereby making it possible to adjust the frequency.
However, with this adjusting method, when the assembled filter is to be contained in an outer casing 9 after the adjustment of the frequency response is made, the removed regions 8 for the frequency adjustment of the upper surface of the upper dielectric substrate 2 come close to or come into contact with the upper wall of the outer casing 9 because the removed regions 8 are positioned on the upper surface of the upper dielectric substrate 2. Therefore, the stray capacitance may be changed from the adjusted value so that the frequency response may deviate. For this reason, if the above mentioned adjusting method is applied, the outer casing should be so designed that it has an inner height larger than the height of the filter assembly and the upper surface of the upper dielectric substrate 2 is sufficiently spaced from the upper wall of the casing 9 as will be seen in FIG. 2. Recently, various equipments or elements adapted for use in a microwave range becomes thinner and it is thus demanded that the filter devices as well as the elements should be constructed in a thinner configuration or dimension.
However, such a demand for a thinner construction can not be satisfied by utilizing the above mentioned adjusting method in which a casing having a larger inner height is necessarily used.